The present invention is directed to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, and more particularly to pacemaker, defibrillator, cardiovertor and/or multisite devices for the treatment of the disorders of the heartbeat. It more particularly relates to the adjustment of the amplitude voltage level of the stimulation pulses over the course of time.
The stimulation pulse amplitude level of the cardiac cavities (ventricle or atrium) is a value typically ranging between 1.5 and 7.5 V, adjustable by step increments of 0.5 V. This amplitude must of course be sufficiently high to cause a depolarization of the myocardium, what is called xe2x80x9ccapturexe2x80x9d. It is, however, necessary to avoid values that are too high to spare the lifespan of the battery. In this regard, the stimulation energy applied, and thus the corresponding energy consumption of the device, is proportional to the square of the amplitude (and also to the duration) of the pulse.
The test for the threshold of stimulation effectiveness, or xe2x80x9cthreshold test,xe2x80x9d can be carried out at regular intervals, for example, every six hours. One such algorithm using an automatic test is described in particular in the WO-A-93/02741 and its corresponding U.S. Pat. No. 5,411,533 commonly assigned herewith to Ela Medical. The stimulation pulse amplitude is then adjusted on the basis of the threshold thus measured, with a large safety margin added: The adjusted level is generally twice the value of the measured threshold, and the adjusted level is limited by a minimum (typically 2.5 V) and by a maximum (typically 5.0 V). This particular adjusted level will be called thereafter xe2x80x9csafety amplitudexe2x80x9d and will be indicated by the labels Vs.
It is an object of the invention to minimize the stimulation pulse voltage, reducing it down to a level that is close to the threshold voltage (that is, the voltage below which there will be no more capture, also known as a loss of capture), by checking of course in a much more frequent way, typically with each cardiac cycle, whether the stimulation was effective so as to readjust the stimulation voltage as far as it can be reduced and/or to switch back to a voltage corresponding to the safety amplitude when appropriate. This reduced stimulation voltage that is close to the capture threshold will be called hereafter xe2x80x9ccapture amplitudexe2x80x9d and will be indicated by the label Vc. This technique is particularly advantageous, because it makes it possible to avoid having to use the large safety margin for the stimulation pulse amplitude, and thus lengthens in a substantial way the lifespan of the battery. On the other hand, insofar as stimulation is made at a level that is close to that of the capture threshold, it is essential to operate a capture test xe2x80x9ccycle to cyclexe2x80x9d, i.e., to examine at each cardiac cycle whether the stimulation pulse was effective rather than to test capture at periodic intervals, for example, every six hours as in the prior known devices. If a loss of capture is detected, a backup-stimulation pulse having a suitably large energy must be immediately applied (i.e., at the end of the 63 ms period following the stimulation that is determined to be ineffective) in order to compensate, without awaiting the absence of a depolarization of the myocardium. Moreover, the next stimulation pulse will be operated on the basis of the aforementioned safety amplitude, and the voltage level of the capture amplitude will be reevaluated to determine whether it is necessary to readjust it to be at a higher amplitude.
It is necessary, however, to take into account the fact that the detection of a loss of capture can actually be only the consequence of the occurrence of a xe2x80x9cfusionxe2x80x9d, i.e., a stimulation intervening in a concomitant way with a spontaneous ventricular depolarization. Indeed, after an atrial stimulation, the detected ventricular event (the well known xe2x80x9cQRSxe2x80x9d complex) can be either the direct result of the stimulation, taking into account the latency time existing between the two events, or a spontaneous QRS complex occurring in the same temporal window (known as a xe2x80x9cfusionxe2x80x9d). The occurrence of a fusion can have a noxious effect from the hemodynamic point of view, because of the presence of two very close myocardial excitations, of which one is useless hemodynamically.
In the case of a capture test, even if a fusion does not have a hemodynamic effect, it is nevertheless likely artificially to produce an increase of the value of the measured capture threshold as compared to the real threshold of the patient, with for a consequence a readjustment of the stimulation amplitude to an excessive level that is maintained at least for several hours. Although this excessive level is not in itself dangerous, it does constitute an unnecessary consumption of battery energy and will reduce the lifespan of the implant.
It is therefore desirable, during the detection of a loss of capture, to discriminate between: a true loss of capture following a natural increase in the capture threshold, the occurrence of a proven fusion, or a simple a typical cycle (post-atrial ventricular detection, too rapid cycle or extrasystole). The last two cases do not justify an immediate revision of the stimulation amplitude level.
The present invention, therefore, is broadly directed to a method and apparatus for determining the capture amplitude according to the detected threshold, taking account of possible occurrence of fusion or a typical cycles, and ensuring if necessary an automatic switch to the safety amplitude.
More particularly, the present invention is directed to an improved device of a known type including: means for stimulating the ventricle by delivering to the heart one or more stimulation pulses, each pulse having a predetermined amplitude and a duration; means for adjusting the amplitude of the stimulation pulse to be delivered; means for evaluating at periodic intervals a capture threshold and for defining a corresponding safety amplitude from the capture threshold; and means for detecting capture cycle to cycle, to determine the detection or the loss of capture on each cardiac cycle after a stimulation at a given stimulation amplitude.
In a characteristic manner of the invention, it is further envisaged to include means for defining a capture amplitude at a level that is function of the capture threshold, and that is equal to or greater than the capture threshold but lower than the safety amplitude, wherein the means for adjusting the stimulation pulse amplitude includes means for reducing temporarily the stimulation amplitude below the safety amplitude and close to the aforementioned capture amplitude value; for checking, immediately after a stimulation using said reduced amplitude, the detection or the loss of capture; and, in the event of a detection of capture, establishing for the next cardiac cycle a stimulation amplitude at the value of the capture amplitude; and, in the event of loss of capture, defining a new stimulation amplitude value that is greater than the then current capture amplitude value.
In a preferred embodiment, the means for defining the capture amplitude establishes the capture amplitude at a level that is equal to the capture threshold increased by a fixed increment, for example, one step of adjustment of the stimulation amplitude.
In a further preferred embodiment, the means for adjusting is operated so that, in the event of a determined loss of capture and before the stimulation amplitude voltage is set at the safety amplitude, it performs a discrimination between, on the one hand, an occurrence of a fusion or an a typical cycle, and, on the other hand, a rise in the capture threshold. The discrimination may in particular be made by a reduction of the escape interval or the atrio-ventricular delay (depending on the pacing mode being used), followed by a detection of capture on a following stimulation operated with a stimulation amplitude corresponding to the capture amplitude.
In a more preferred embodiment, the means for adjusting the stimulation amplitude value is operated, in the event of the occurrence of a fusion or of an a typical cycle, to perform an additional discrimination between, on the one hand, the occurrence of a fusion, and, on the other hand, the occurrence of an a typical cycle. Such a discrimination may be made, in particular, by lengthening the escape interval or the atrio-ventricular delay, followed by detecting capture on a consecutive stimulation operating with a stimulation amplitude value corresponding to the safety amplitude. In the event of fusion, the stimulation amplitude is preferably established at the value of the safety amplitude and, in the event of a determined a typical cycle, the re-establishment of the capture is detected over a predetermined number of successive consecutive cycles and, in such a case, the stimulation amplitude is established at the value of the capture amplitude for the next cardiac cycle.